But, for three days starting last Sept. 11, meteorological researchers were presented with just such an opportunity when the FAA grounded commercial flights nationwide for three days following the terrorist air attacks.

And now it has emerged that the American climate was indeed noticeably different during those three days without air travel.

A team of climatologists presented their work Tuesday, showing that temperatures in the United States fluctuated by 1.2 degrees Celsius more when airplanes were grounded than when normal flight patterns prevailed. That is, planes in the sky dampen the variability between day and nighttime temperatures. More air travel, the researchers suggest, brings less meteorological difference between noon and midnight.